Transistors, when used to drive inductive loads in the switching mode, can be damaged by the voltage surge associated with the turn off operation. While the transistor could normally dissipate the energy involved, the breakdown mechanism tends to localize the action. This creates a hot spot in the transistor which in turn further localize the action and burn out occurs at the hot spot. In general when a voltage surge is applied to a transistor, electrical breakdown occurs at that point where the collector most closely approaches the emitter. The resultant current flow is localized and intense local heating at the point of breakdown further localizes the action. Thus a surge of relatively low energy can burn out a transistor of substantial power handling capability.
One approach to the problem has been to arrest the voltage surge in the inductive load with a separate device. For example a large capacitor can be connected across the load so that voltage surges are filtered. Alternatively a diode, poled to conduct during the surge, can act to short circuit the surge. Both of these approaches produce a very slow decay of surge energy and result in an often unacceptably long switching time.
A second approach is to connect a zener diode between the collector and base of the switching transistor. The zener voltage is selected to be as high as possible but lower than transistor breakdown voltage. Using this arrangement results in the voltage surge being arrested by the transistor but while it is active and thus capable of handling a large current distributed over its full area. The action also occurs at a relatively high voltage so that rapid decay of the surge can be achieved. However, it is difficult to match a zener diode with a transistor. The prior art practice has been to use a plurality of low voltage zener diodes in a stack with a single switching transistor. Typically the zener diodes are made using conventional transistor emitter-base structures. Thus the zener diodes are on the order of six volt units. To protect a 60-volt transistor a stack of nine such units would be series connected. Such an arrangement works well but the large number of diodes is cumbersome and expensive. Furthermore there is a problem in that the zener voltage does not track transistor breakdown in terms of the temperature of the switching transistor collector.